setup_arch (2) - Kernel boot arguments_parse_early_param()

[Bootloader傳什麼進入kernel]
Bootloader 會傳入一串參數給kernel 以決定kernel 的運行
傳給核心的參數是以空格分隔的字串，通常的型式是
param[=value_1][,value_2]...[,value_10]

param是keyword, 一個param後面可以接最多10個value

由 bootloader 傳給核心的參數字串也可以包含傳給 init 程序的參數，kernel只會解析到 "--" 之前的字串，在 "--" 之後的字串會被當成傳給 init 程序的參數

[Bootloader用什麼data struct 傳入kernel]

使用struct tag, define 如下

 struct tag {
         struct tag_header hdr;
         union {
                 struct tag_core         core;
                 struct tag_mem32        mem;
                 struct tag_videotext    videotext;
                 struct tag_ramdisk      ramdisk;
                 struct tag_initrd       initrd;
                 struct tag_serialnr     serialnr;
                 struct tag_revision     revision;
                 struct tag_videolfb     videolfb;
                 struct tag_cmdline      cmdline;
 
                 /*
                  * Acorn specific
                  */
                 struct tag_acorn        acorn;
 
                 /*
                  * DC21285 specific
                  */
                 struct tag_memclk       memclk;
         } u;
 };

每一個tag都有一個tag_header定義如下

  struct tag_header {
          __u32 size;
          __u32 tag;
  };

[When will Bootloader pass into kernel]

 void __init setup_arch(char **cmdline_p)
 {
         pr_info("Boot CPU: AArch64 Processor [%08x]\n", read_cpuid_id());
 
         sprintf(init_utsname()->machine, ELF_PLATFORM);
         init_mm.start_code = (unsigned long) _text;
         init_mm.end_code   = (unsigned long) _etext;
         init_mm.end_data   = (unsigned long) _edata;
         init_mm.brk        = (unsigned long) _end;
 
         *cmdline_p = boot_command_line;
 
         early_fixmap_init();
         early_ioremap_init();
 
         setup_machine_fdt(__fdt_pointer);
 
         parse_early_param();
 ...

從start_kernel --> setup_arch --> parse_early_param
進到parse_early_param就會開始解析preloader傳進來的參數

[early_param]
有較高優先權的參數稱為early_param
以maxcpus為例, 由preloader 傳給kernel, 因為(when __SMP__ is defined)boot up 時就需要知道maxcpus是多少, 因此在early_param時就會執行 maxcpus() in kernel/smp.c
More 可以參考
http://man7.org/linux/man-pages/man7/bootparam.7.html

 static int __init maxcpus(char *str)
 {
         get_option(&str, &setup_max_cpus);
         if (setup_max_cpus == 0)
                 arch_disable_smp_support();
 
         return 0;
 }
 
 early_param("maxcpus", maxcpus);

至於early_param 這個macro又是什麼呢? 繼續往下看
include/linux/init.h

 /*
  * Only for really core code.  See moduleparam.h for the normal way.
  *
  * Force the alignment so the compiler doesn't space elements of the
  * obs_kernel_param "array" too far apart in .init.setup.
  */
 #define __setup_param(str, unique_id, fn, early)                        \
         static const char __setup_str_##unique_id[] __initconst         \
                 __aligned(1) = str;                                     \
         static struct obs_kernel_param __setup_##unique_id              \
                 __used __section(.init.setup)                           \
                 __attribute__((aligned((sizeof(long)))))                \
                 = { __setup_str_##unique_id, fn, early }
 
 #define __setup(str, fn)                                                \
         __setup_param(str, fn, fn, 0)
 
 /*
  * NOTE: fn is as per module_param, not __setup!
  * Emits warning if fn returns non-zero.
  */
 #define early_param(str, fn)                                            \
         __setup_param(str, fn, fn, 1)

early_param 跟 __setup 的實作都是 __setup_param, 只不過 early_param --> __setup_param會將其中的early參數設為1

進入__setup_param會發現這個macro其實是define兩個variable
(1) define 一個型態為 static const char 的variable, 並將str assign給它
     __setup_str_##unique_id[] = str

(2) define 一個型態為 static struct obs_kernel_param 的variable __setup_##unique_id
      並初始化data structure
      __setup_##unique_id = {__setup_str_##unique_id, fn, early}

struct obs_kernel_param 可以讓kernel記錄字串參數與相對應的處理函式
一樣定義在 include/linux/init.h

 struct obs_kernel_param {
         const char *str; //name of params
         int (*setup_func)(char *); // function handler
         int early; //true if it's early_param
 };

我們再一次用上面maxcpus來展開

 static int __init maxcpus(char *str)
 {
         get_option(&str, &setup_max_cpus);
         if (setup_max_cpus == 0)
                 arch_disable_smp_support();
 
         return 0;
 }
 
 early_param("maxcpus", maxcpus);

macro 炸開之後........

static const char __setup_str_maxcpus[] __initconst __aligned(1) = "maxcpus"

static struct obs_kernel_param __setup_maxcpus __used __section(.init.setup) __attribute__((aligned(sizeof(long)))))

= { __setup_str_maxcpus, maxcpus, 1};

這樣就可以很清楚的發現
我們定義了一個裝有"maxcpus"的字串陣列, 再來有一個記錄 kernel params的 struct obs_kernel_param, 用來告訴kernel 這個param叫什麼, 以及遇到這個param的時候開call 什麼fundtion handler來處理

另外用藍色標起來的部分又是一個難題了
這裡將這個macro定義的資料放在 .init.setup 這個section中
關於.init.setup 這個section 是定義在 kernel-4.4\include\asm-generic\vmlinux_lds.h 中

#define INIT_SETUP(initsetup_align) \
. = ALIGN(initsetup_align); \
VMLINUX_SYMBOL(__setup_start) = .; \
*(.init.setup) \
VMLINUX_SYMBOL(__setup_end) = .;

代表所有被標註要放到.init.setup這個section的params都會被放在一起
而起始為至是 __setup_start & __setup_end

怎麼知道到底有哪些params會被放在 .init_setup中呢
可以打開 out\System.map看看
找到__setup_start了!! 下面緊接著都是 __setup_XXX
還看到了前面的舉例 __setup_maxcpus 這個param

[parse_early_param]
回到start_kernel--> setup_arch--> parse_early_param
這裡將boot_command_line copy 給 tmp_cmdline
接著呼叫 parse_early_options--> parse_args 開始解析kernel parameters
parse_args 會將cmd args切成一組組的<param,value>, 傳入callback function do_early_param

 void __init parse_early_options(char *cmdline)
 {
         parse_args("early options", cmdline, NULL, 0, 0, 0, NULL,
                    do_early_param);
 }
 
 /* Arch code calls this early on, or if not, just before other parsing. */
 void __init parse_early_param(void)
 {
         static int done __initdata;
         static char tmp_cmdline[COMMAND_LINE_SIZE] __initdata;
 
         if (done)
                 return;
 
         /* All fall through to do_early_param. */
         strlcpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE);
         parse_early_options(tmp_cmdline);
         done = 1;
 }

[do_early_param]
將傳入的params 跟 __setup_start , __setup_end區間中的args做比對, 若有符合的, 就call 該args的callback function (p->setup_func)做設定

 /* Check for early params. */
 static int __init do_early_param(char *param, char *val,
                                  const char *unused, void *arg)
 {
         const struct obs_kernel_param *p;
 
         for (p = __setup_start; p < __setup_end; p++) {
                 if ((p->early && parameq(param, p->str)) ||
                     (strcmp(param, "console") == 0 &&
                      strcmp(p->str, "earlycon") == 0)
                 ) {
                         if (p->setup_func(val) != 0)
                                 pr_warn("Malformed early option '%s'\n", param);
                 }
         }
         /* We accept everything at this stage. */
         return 0;
 }

reference:
[1] https://danielmaker.github.io/blog/linux/kernel_parameter_parsing.html
[2] http://blog.csdn.net/goto_chen/article/details/17392245
[3] http://blog.csdn.net/skyflying2012/article/details/41142801